This invention relates to a miniaturized nuclear battery, composed of several cells mounted in series.
Such sources of electricity are needed, so that electrically driven apparatus, as for example electric or electronic watches, can operate without interruption caused by the periodic replacing of the batteries.
Batteries as mentioned above are already known, but satisfying results could not be obtained for a miniaturized nuclear battery, fulfilling all conditions relative to security, a life of 10 to 20 years, and an appropriate output power.
In particular such batteries are known which contain higher energetic .beta.-sources as Sr 90, Pm 147, etc. For safety reasons such batteries are not suited for uncontrolled use.
Because of the limited intensity of the radiation for safety and in particular because of its half-life of 12.5 years, tritium (T or .sup.3 H) appeared to be singularly suited as a source of radiation, in spite of the fact that only vacuum could be used as dielectric medium. This is a disadvantage vis-a-vis the mentioned higher energetic .beta.-sources, as they could have allowed a solid, and thus a more dense dielectric with a half-value-thickness for these emitters far above the 40 mg/cm.sup.2 for tritium. Due to the medium intensity of radiation of about 6 keV of the tritium, batteries with this source are only feasible with vacuum as dielectric. Although by more recent embodiments of such batteries the freed helium no longer lowers the vacuum in a short time, there exist no results wherein a vacuum of 10.sup..sup.-3 Torr is maintained as long as the life of the battery lasts, without losses.
Another drawback of the vacuum battery turns up when a satisfying amount of current has to be furnished, which has to be done by increasing the flow of electrons to the collector and that means, as the surface activity has to be limited, an increase of the total surface. This is reached by a special arrangement of the emitter surface, which increases the volume of the battery. For the application of such batteries in miniaturized devices as for example watches, hearing aids, pace-makers, etc., the dimensions play an eminent role, often tenths of millimeters are most important.
An impedance matching of such batteries to the devices to be supplied is very difficult as their no-load voltage corresponds practically to the mean radiation energy of tritium. With a matched load, U.sub.eff is still 2-3 kV.